Generally, a rotation angle detector is realized as an angle sensor disclosed in, for example, a patent document, Japanese Patent No. 5339094 (Patent document 1) shown below. This angle sensor is a sensor used in the electric power steering device, and detects a steering angle of the steering wheel. The angle sensor is provided with a motor rotation angle sensor for detecting a rotation angle and the number of rotations of a motor in the electric power steering device. The steering angle of the steering wheel is then calculated from the detection result of the motor rotation angle sensor. When a supply of voltage from an in-vehicle battery to the motor rotation angle sensor is continued even during an ignition switch OFF time, the steering angle, or more precisely, a change of the steering angle, of the steering wheel is continuously detectable.
In terms of vehicle control, the steering angle of the steering wheel needs to be detectable only during an ignition switch ON time. Therefore, during the ignition switch OFF time, only the number of rotations of the motor needs to be detectable. Further, the supply of voltage from the in-vehicle battery to the motor rotation angle sensor is configured to be intermittent for power-saving purposes. However, such an intermittent supply of voltage may cause an inaccurate rotation number sensing (e.g., the sensing of the rotation number may become intermittent and may yield a discontinuously false sensing result). In other words, by increasing the operation frequency of (i.e., the power supply frequency to) the motor rotation angle sensor, the sensing accuracy may be improved, but the power consumption may also be increased at the same time.
In view of the above situation, a steering angle detector for detecting a steering angle of a steering wheel in an electric power steering device is proposed, which is equipped with (i) a rotation angle detection section for detecting a rotation angle in the electric power steering device and (ii) a rotation number detection section for detecting a number of rotations of the motor in the electric power steering device.
During an ignition switch ON time, the voltage supply is provided from the in-vehicle battery to the rotation angle detection section, and regardless of the ignition switch ON-OFF, the voltage supply is provided from the in-vehicle battery to the rotation number detection section. In such configuration, the rotation angle detection section receives the voltage supply only during the ignition switch ON time, thereby achieving a more limited operation of the detection section, in comparison to the configuration of the patent document 1. That is, the power consumption of the steering angle detector is further reduced in such circuit configuration, while limiting the operation of the steering angle detector.
However, with the above-described steering angle detector, the voltage supply from the in-vehicle battery to the rotation number detection section is continued regardless of the ON or OFF positions of the ignition switch. Thus, in case of lowering or interruption of the voltage supply from the in-vehicle battery, the rotation number detection section of the steering angle detector cannot detect the number of rotations of the motor in the electric power steering device. Therefore, if the steering wheel is operated in such a situation (i.e., during a voltage supply lowering/interruption period), the steering angle of the steering wheel cannot be accurately detected.